The present invention relates to an explosive detection system used to detect explosives in checked airline baggage. More specifically, the present invention relates to a nuclear-based explosive detection system that accurately discriminates between parcels with and without explosives. The system includes, in addition to a neutron source and an array of gamma ray detectors, at least one neutron detector, means for removing background noise from the ensuing gamma ray spectrum, an X-ray system, and/or an artificial neural system (ANS).
A great need exists for the scanning of luggage, baggage and other parcels for the detection of any explosive material contained or concealed within their confines. For example, a large number of pieces of luggage (estimated at over 2,000,000) are checked and/or carried onto aircraft daily by close to seven hundred and fifty thousand (750,000) passengers within six hundred (600) airports extending across the country. Many more packages move through the mails or are shipped to sensitive buildings. There is a possibility, albeit small, that any one piece of luggage or parcel might contain explosive material. It is, therefore, desirable to protect the public by providing detection systems to scan the luggage and parcels to detect the presence of any explosive material.
It thereby follows that any system of checking luggage or parcels must have a very high probability of detection (PD) in order to be effective. Because of the large number of parcels processed, a high throughput is necessary for practicality. In addition, any detection system, because of the large number of scanned items, is bound to occasionally give a false alarm. The probability of these false alarms (PFA) must be minimized in order to provide for an effective explosive detection system. This is because when an alarm occurs it is not known, at that time, whether it is true or false. This means that each time an alarm occurs, a passenger or parcel must be detained for further investigation. If false alarms are significantly high the nuisance level and the delays could be unacceptable to the public. It is, therefore, important that any explosive detection system must have a very high probability of detection (high PD), a high throughput rate, and yet at the same time have a very low probability of false alarms (low PFA). These conflicting criteria have hampered efforts in the past to build a reliable and usable system.
In general, prior art systems have not met the desired characteristics of having a high probability of detection (PD) with a low probability of false alarms (PFA) at acceptable throughput rates. As an example, one such prior art system is shown in U.S. Pat. No. 3,832,545. This patent provides for a system for the detection of nitrogen, which is generally present in the explosive materials to be detected. As described in the referenced patent, a rough two-dimensional profile of the nitrogen content within the object being inspected is provided. This profile is then used in an attempt to determine whether explosive materials are present. Unfortunately, however, because of the types of detectors used by the invention described in the '545 patent (liquid or plastic scintillators), the processing of the detector signals is quite inefficient. Moreover, the two-dimensional limitation allows many materials to be positioned in the object being examined so as to defy detection, thereby providing an unacceptably low PD and high PFA.
Other types of prior art explosive detection systems depend upon the prior seeding of explosive materials with a tracer material, such as a radioactive tracer. Although this type of system could be very useful if all explosive material were manufactured with such tracer material, because of the large amount of explosive material which has already been manufactured and because of the difficulty of controlling the manufacture of all explosive material so as to contain such tracer material, this type of system is not practical. A useable system must be able to detect the presence of explosive material of a conventional type and of an unconventional type, whether disposed within an object either in its original manufactured form, or if deployed within the object so as to attempt to confuse or evade the detection system. The prior art systems have not met these various criteria and cannot produce the desired high probability of detection with the relatively low production of false alarms.
An acceptable response to the explosive threat to aviation, mails, or shipping requires detection techniques that are highly sensitive, specific, rapid and non-intrusive. The efficient detection of nitrogen, at this point, offers the best overall solution, although other elements, such as hydrogen and chlorine, could also be detected. It is, therefore, important that the detection of nitrogen be provided to give the maximum information of the physical parameters of the explosive, such as density and spatial distribution. The use of nuclear based techniques which subject the luggage or parcels to thermal neutrons can be the basis of a system to produce the desired results, but this system cannot be based on the prior art techniques. It is important that the intensity, energy and spatial distribution of the detected radiations from the object under observation be provided in a way to help determine the presence or absence of explosives, and this has not yet been accomplished.
In addition to high detection sensitivity and low false alarm, the detection of the explosive should be independent of the specific configuration and must be non-intrusive in order to protect privacy. The detection equipment, of course, must be non-hazardous to the contents of the checked items and to the operating personnel and environment. Other more general criteria are that the system must be reliable, easily maintained and operable by relatively unskilled personnel and that the cost must be low enough so as to be non-burdensome to airlines and airports. Finally, it is desirable, when all other requirements are achieved, that the size of the system be relatively small so that the system may be useful in a wide variety of environments.
In addition to the nuclear based systems described above, non-nuclear systems have also been investigated. These systems have occasionally achieved relatively high efficiencies of detection for some types of explosives, but generally have relatively high false alarm rates and have long screening times. These type of non-nuclear systems, therefore, by themselves cannot achieve the desired results. However, some features of such non-nuclear systems may advantageously be combined with a nuclear system as described herein, to significantly improve the overall detection capabilities of the system.
In order to develop a proper explosive detection system, an understanding is required of the properties of the various explosives relevant to the specific techniques to be used. Although there are a large number of explosive types, a general classification into six major groups with minor variations, has been proposed. The proposed classification scheme includes the following types of explosives: (1) nitroglycerine based dynamites, (2) ammonium nitrate based dynamites, (3) military explosives, (4) homemade explosives, (5) low order powders, and (6) special purpose explosives.
In general, all of these explosive types contain a relatively high amount of nitrogen ranging from nine to thirty five percent by weight. The nominal density of these explosives is typically 1.6 g/cm.sup.3, with ranges between 1 to 2 g/cm.sup.3 or more. These physical properties demonstrate that the most unique signature of explosives is the high concentration and density of the nitrogen content. Also the presence of other elements in combination with the presence of nitrogen can be considered. Physical factors may also help identify explosives. One physical factor, for example, is a minimum propagation thickness or diameter in order for most explosives to be effective. This minimum propagation thickness requires a sizeable contiguous body of explosives in the other two dimensions. This information is useful to the detection of explosives without making a specific assumption of the actual shape of the explosive.
It can be seen, therefore, that a nuclear detection technique can provide for the detection of the nitrogen content, which can provide an indication as to the presence of an explosive. However, the frequent occurrence of nitrogen in non-explosive materials limits the level of detection sensitivity and merely detecting the presence of absence of nitrogen alone is generally not sufficient. Therefore, additional information is required beyond simply sensing the presence of the nitrogen. The present invention provides for this additional information.